NMR (nuclear magnetic resonance) or MRI (magnetic resonance imaging) is a method for measuring the structure and property of an examination object with the use of an atomic nucleus as a probe, and has been widely used in the medical field and inspection field (Nonpatent Literature 1). The frequency of the nuclear magnetic resonance signal is proportional to the intensity of a static magnetic field, and the proportionality coefficient is a constant peculiar to an atomic nucleus, and is referred to as the gyromagnetic ratio of the nucleus. For example, the gyromagnetic ratio of a proton 1H is 42.58 MHz/T (tesla). Generally, as the intensity of the static magnetic field becomes stronger, the sensitivity and the resolution of the measurement become more improved; therefore, the intensity of the static magnetic field currently used for the NMR/MRI is equal to 1 T or larger. Therefore a static magnetic field coil used for the NMR/MRI is comprised of superconducting wires.
Generally, an induction coil is used for detecting a nuclear magnetic resonance signal. The signal intensity obtained by the induction coil scheme is proportional to the product of the magnetic moment Mo and the resonant angular frequency ω of a sample. Mo and ω are respectively proportional to the intensity of a static magnetic field Bo, therefore the signal intensity is proportional to the square value of Bo. Therefore, if the intensity of the static magnetic field is small, the signal intensity becomes extremely small.
On the other hand, a measurement for measuring NMR/MRI in the region of an intensity of static magnetic field 10 μT to 100 μT, which is of the geomagnetic intensity level, that is, a measurement for measuring an ultralow magnetic field NMR/MRI has been widely attracting attention in recent years. The frequency of the nuclear magnetic resonance signal of a proton in the ultralow magnetic field NMR/MRI is about 1 kHz to 10 kHz, and this frequency is five to six digits lower than a frequency that is used in an ordinary magnetic field NMR/MRI. Therefore, the measurement in the ultralow magnetic field NMR/MRI is characterized in that, in order to obtain a high signal intensity, pre-polarization is performed on a nuclear spin of a measurement object with the use of a polarized magnetic field of about 10 mT to 100 mT, and a SQUID (superconducting quantum interference device: Nonpatent Literature 2), which is a high-sensitive magnetic sensor, is used instead of the induction coil (Nonpatent Literature 3).
Because a static magnetic field coil and a gradient magnetic field coil used in the ultralow magnetic field NMR/MRI are comprised of room-temperature cupper wire coils, these coils can be drastically downsized, can be made low cost, and can be improved in their safety aspect in comparison with a superconducting magnet. In addition, because the signal frequency decreases to the order of several kHz, the NMR/MRI can be performed in the presence of a metal body.